Not to say the ability is broken because any bit of magic resist can make it feel very underwhelming at times.
It's just that the ability tends to hold mid Morgana back because when Morgana becomes especially strong in mid she kinda just shoves the wave over, and over again. Unlike support Morgana, mid Morgana is rather uninteractive because her W clears casters on its own by rank 3.
Ideally it'd be changed into something potentially functional for both support/mid, mostly mid, but I can see how that'd be difficult to do given how stretched her power budget already is. Mind you I say none of this because I hate playing against Morgana mid(Haven't played against one in eons), it's because I find Morgana mid to quite the enjoyable experience, but it isn't uncommon for me to resort to non stop shoving once I hit level 5.